Designed Single-Step Synthesis, Structure, and Derivative
Textural Properties of Well-Ordered Layered Pentacoordinate Silicon Alcoholate Complexes
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Citation
Li, Xiansen, Vladimir K. Michaelis, Ta-Chung Ong, Stacey J.
Smith, Robert G. Griffin, and Evelyn N. Wang. “Designed SingleStep Synthesis, Structure, and Derivative Textural Properties of
Well-Ordered Layered Penta-Coordinate Silicon Alcoholate
Complexes.” Chem. Eur. J. 20, no. 21 (May 19, 2014):
6315–6323.
As Published
http://dx.doi.org/10.1002/chem.201400379
Publisher
John Wiley & Sons, Inc
Version
Author's final manuscript
Accessed
Wed May 25 23:38:39 EDT 2016
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http://hdl.handle.net/1721.1/87685
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Designed Single-Step Synthesis, Structure, and Derivative Textural Properties of
Well-Ordered Layered Penta-Coordinate Silicon Alcoholate Complexes
Xiansen Li,*[a] Vladimir K. Michaelis,[b,c] Ta-Chung Ong,[b,c] Stacey J. Smith,[b] Robert G.
Griffin,[b,c] and Evelyn N. Wang*[a]
[a] Dr. X. Li, Prof. E. N. Wang, Department of Mechanical Engineering, Massachusetts Institute of
Technology, 77 Massachusetts Ave., Cambridge, MA 02139, (USA)
E-mail: xsli@mit.edu; enwang@mit.edu.
[b] Dr. V. K. Michaelis, T.-C. Ong, Dr. S. J. Smith, Prof. R. G. Griffin, Department of Chemistry,
Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, (USA).
[c] Dr. V. K. Michaelis, T.-C. Ong, Prof. R. G. Griffin, Francis Bitter Magnet Laboratory, Massachusetts
Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, (USA).
Abstract: The controllable synthesis of well-ordered layered materials with specific nanoarchitecture
poses a grand challenge in materials chemistry. We report the solvothermal synthesis of two structurally
analogous 5-coordinate organosilicate complexes via a novel transesterification mechanism. Since the
polycrystalline nature of the intrinsic hypervalent Si complex thwarts the endeavor in determining its
structure, a novel strategy concerning the elegant addition of a small fraction of B species as an effective
crystal growth mediator and a sacrificial agent is proposed to directly prepare diffraction-quality single
crystals without disrupting the intrinsic elemental type. In the determined crystal structure, two
monomeric primary building units (PBUs) self-assemble into a dimeric asymmetric secondary BU via
strong Na+-O2- ionic bonds. The designed one-pot synthesis is straightforward, robust, and efficient,
leading to a well-ordered (10ī)-parallel layered Si complex with its principal interlayers intercalated with
extensive van der Waals gaps in spite of the presence of substantial Na+ counter-ions as a result of unique
atomic arrangement in its structure. On the other hand, upon fast pyrolysis, followed by acid leaching,
both complexes are converted into two SiO2 composites bearing BET surface areas of 163.3 and 254.7 m2
g-1 for the pyrolyzed intrinsic and B-assisted Si complexes, respectively. The transesterification
methodology merely involving alcoholysis but without any hydrolysis side reaction is designed to have
generalized applicability for use in synthesizing new layered metal-organic compounds with tailored
PBUs and corresponding metal oxide particles with hierarchical porosity.
Keywords: boron-mediated synthesis • hypervalent compounds • layered compounds • modified
transesterification • van der Waals gap
1
Introduction
In contrast to the ubiquitous tetrahedrally coordinated silicate species, the reports of 5-coordinate Si ([5]Si)
species have so far been infrequent in alkoxide-based crystalline solids.[1-6] Consequently, there are still
some unknown aspects about their physicochemical properties. The laborious preparations of several
[5]
Si
complexes were disclosed in multiple synthetic steps[1,3] by reacting SiO2 grains with alkali hydroxide in
excessive ethylene glycol (EG) solvent with continuous distillation of EG and coincident distillative
removal of liberated water to drive the dissolution process to proceed smoothly. Additionally, this
technique encountered severe foaming in the presence of Na+ cations over the course of the synthetic
process. More importantly, water-sensitive or hygroscopic silicoalcoholate complexes cannot be produced
by this method due to the unavoidable water interference therein. Although the conventional
transesterification reaction involving carboxylic acid esters has already been well-exploited in organic
synthesis, e.g., biodiesel production from triglyceride and light alcohols,[7] it is not applicable for the
preparation of metal-organic compounds (e.g., layered 5-coordinate organosilicates) without any
modification of the reaction.
Crystallography and NMR are the most powerful techniques available for unraveling unknown
molecular structures. However, neither polycrystalline powders nor single crystals of 5-coordinate
alkoxysilicates with mixed methanolate/glycolate ligands have been thus far disclosed, not to mention
their crystal structure determination[1,3a]. In general, growing large single crystals indispensable for
crystallography can be classified into two major categories: direct and indirect methods. In the latter case,
it manages to transform the polycrystalline powders into the respective single crystals by time-intensive
recrystallization techniques provoked primarily by controlled solvent evaporation, slow cooling, vapor
diffusion, liquid-liquid inter-diffusion, sublimation, and convection, etc. Another disadvantage associated
with the indirect method is that the recrystallized single crystals are occasionally phase transformed.[8]
Particularly, the effective preparations of crystallographically resolvable single crystals are a significant
challenge for Si-containing compounds owing to the strong propensity of silicates towards fast gelation
even at the reactant mixing stage. As a general countermeasure against the undesirable gelling, excessive
strong chelators and/or sparingly soluble silica sources, e.g., quartz, are admixed together with dilute
alkaline solutions to regulate the relative contribution between nucleation and crystal growth,[9-11] thereby
causing dramatically extended synthetic duration and aggravated waste disposal loads.
The versatility of well-ordered layered materials makes them quite attractive for several important
applications including lithium-ion batteries,[12] supercapacitors,[13] drug delivery,[14] adsorbents,[15,16]
catalysts,[17] and catalyst supports.[18] Among the known layered materials, those with their interlayers
intimately linked together only by long-range attractive force of van der Waals type (i.e., so-called van
der Waals gap as typically found in the FeOCl[12a] and TiS2[12c]) are quite rare in the presence of charge2
balancing alkali metal counter-ions. This feature could be quite useful for advanced lubricants and
precursors towards preparing nanoflake/polymer composite membranes for gas-/liquid-mixtures
separations.
In this contribution, we report a novel single-step solvothermal preparative methodology, i.e., a
modified transesterification mechanism, for preparing two layered
[5]
Si complexes with a similar
asymmetric unit. In addition, a new B-mediated approach that does not involve any B incorporation into
the intrinsic host silicate matrix is formulated herein for the facile synthesis of large-sized single crystals
of the hitherto poorly-characterized binary-ligand
[5]
Si complex, with which its crystal structure is for the
first time determined in this study. Also, we demonstrate a proof-of-concept study regarding the rapid
pyrolytic conversion of these Si complex precursors into high-surface-area amorphous SiO2 composites
with minor residual carbon loadings.
Results and Discussion
Solvothermal synthesis: A modified transesterification strategy using pseudo-covalent metal alkoxide
ester instead of the traditional carboxylic acid ester is proposed for the one-pot synthesis of two wellordered layered [5]Si complexes under anhydrous circumstances. Over the course of solvothermal reaction
involving B mediator, the effective deprotonation of EG reactant (only 1.5-fold excess from
stoichiometry) is initially accomplished by the operative strong base catalyst of excessive sodium
methoxide to initiate the SN2 nucleophilic substitution reaction, leading to the formation of ethylene
glycolate anions. These incoming anions thus formed then compete with stoichiometric amounts of
methanolates for nucleophilic attack of the Si electrophiles, thereby causing the concurrent departure of
CH3O- anions from Si(OCH3)4 reactant. Due to the chelating nature of the bidentate glycolate, Si
electrophile obviously possesses a coordinative preference for glycolate over monodentate methanolate,
thus affording a unique
[5]
Si complex at an ultimate methanolate/glycolate ratio of 1:2 in the empirical
formula of the B-assisted Si complex (Table S1). The designed transesterification mechanism is
advantageous because it only involves alcoholysis but precludes any hydrolysis side-reaction, thereby
effectively eliminating the coincident generation of metal hydroxide precipitate as a contaminant. It also
offers the opportunity and flexibility for designing novel moisture-sensitive or hygroscopic compounds.
It is worth noting that initial studies were dedicated to detailed characterization of the intrinsic
[5]
Si
complex without any B pre-dosing. However, this attempt was thwarted by the failure in directly
achieving X-ray quality single crystals. Inspired by the intriguing borosilicate glass and zeolite
chemistries,[19-21] where B heteroatom shows the difficulty in isomorphously inserting in considerable
proportion into the silicate network matrix due to the substantial mismatching between d(Si-O) (1.58-1.64
Å)[22] and d(B-O) (1.44-1.52 Å) both in 4-coordinate geometries,[23] an efficient strategy, by which
3
crystallographically resolvable single crystals are successfully achieved by taking advantage of Bmediated transesterification synthesis, is developed herein in order to address the hitherto unknown
crystal structure of the [5]Si complex bearing mixed alcoholates. It is found that B(OCH3)3 additive allows
for a well-mobile thin gel precursor formulation rather than the instantaneous creation of an undesirable
sluggish thick gel entity in the absence of B mediator. In this case, the vast majority of B species are
supposed to still partition in the dynamically equilibrated bulk solution in close contact with the gel phase.
The dynamic bi-phasic B distribution effectively inhibits diverse Si species from full condensation into a
thick gel mass that is detrimental for large single crystal growth. It is well-known that B forms diester
complexes with polyol chelators more readily than does Si. It therefore seems likely that organosilicate
crystal growth is seeded by rapidly created bis(glycolato)borate crystal nuclei. When the crystallization
process progresses, B would be gradually expelled from the growing crystal lattice because it cannot
support 5-coordination unlike Si. The hypothetical [5]B species isomorphously substituting for [5]Si sites is
highly energetically unfavorable due to the large negative charge density (i.e.,
[5] 2-
B ) localized on the
small B center, preliminarily validating the lack of detectable B insertion into the 5-coordinate
organosilicate matrix. Hence, this B-mediated synthetic methodology proposed here provides a unique
opportunity to study in depth Si chemistry by rationally growing the relevant single crystals.
PXRD analysis: Experimental powder X-ray diffraction (PXRD) patterns of the intrinsic Si complex
(A), B-assisted counterpart (B), and simulated PXRD pattern of a single crystal of the latter (C) are
graphically stacked in Figure 1. Both samples are highly crystalline, as evidenced by negligible
background intensities and very sharp major peaks. The zoomed-in profiles in the high-angle regime
(Figure S1 A and B) reveal that following B dosing (5.3% relative to Si), the XRD pattern differs to some
extent from that of the intrinsic Si complex in terms of peak 2θ position and relative intensity.
Interestingly, regardless of the preparative methodology, the intrinsic Si complex exhibits the XRD
pattern analogous to those reported in Refs.[2,3] for Na2Si2(OCH2CH2O)5 complex but with the most
intense peak shifting from 11.02° 2θ for the former down to 10.81° 2θ for the latter, presumably
originating from the swelling effect of EG solvent adopted in the latter syntheses. Namely, they are
isostructural with each other. Combining B and C in both Figures 1 and S1, the experimental PXRD
pattern of the B-assisted Si complex closely matches the simulated pattern derived from the single-crystal
XRD (SCXRD) structure, manifesting the high phase purity of the well-developed single crystalline
grains. The strongest peak corresponding to (10ī) reflection at 11.27° 2θ (7.84 Å in interplanar d-spacing)
is indicative of a well-organized layered B-assisted Si complex with its dominant interlayers parallel to
the (10ī) plane (Figure 1 B).
After rapid pyrolysis of both Si complexes at 800 °C for 8-10 min, the resultant samples of Si_py and
Si+B_py are proven X-ray amorphous, regardless of B dosing (data not shown).
SCXRD,
29
Si and 13C MAS NMR analyses: The molecular geometry and atom-labeling scheme for
4
the B-assisted Si complex are illustrated in the thermal ellipsoid plot of Figure 2. The complex
crystallizes in the monoclinic crystal system and the space group P21/n (Table S1) with the asymmetric
unit
containing
one
dimeric
sodium
tetrakis(glycolato)bis(methanolato)silicate
complex
Na2Si2(OCH3)2(OCH2CH2O)4. Both Si centers are uniquely 5-coordinate; each is bound by two bidentate
glycolate chelators and one monodentate methanolate ligand. 29Si magic angle spinning nuclear magnetic
resonance (MAS NMR) (Figure 3 A) reveals two distinct
[5]
Si resonances located at -105 and -108 ppm
further supporting the assignment of two crystallographic Si sites.[2,3a,4] The
13
C{1H} MAS NMR
spectrum (Figure 3 B) shows two well-resolved regions which can be assigned to the ethylene glycolate
(63 ppm) and methanolate (53 ppm) ligands.[24] The carbon ratio between the glycolate and methanolate
sites using both direct excitation and cross polarization (CP) gives ca. 4:1, corroborating the monomeric
BU structure determined by SCXRD studies. Since the carbon atoms (C5 and C6) of one of the four
glycolates are position-disordered, we speculate the disorder of the glycolates is the cause of the extra
shoulder resonances observed in the
13
C MAS NMR spectrum (i.e., 66 and 54 ppm in Figure 3 B).
Similarly, two discernible regions are also observed in the
13
C{1H} MAS NMR spectrum of the B-
undoped Si complex Na2Si2(OCH2CH2O)5 (Figure 3 C) with the least intense resonance at 52.5 ppm
probably attributable to a very small number of the unbound MeOH solvent molecules tightly occluded in
the interplanar voids, as found in Ref.[24]. There remain three resolvable resonances in the bound glycolate
domain with an intensity ratio of ca. 1:2:2. Among them, the weakest resonance is assigned to the sole
bridging glycolate ligand with the other two resonances corresponding to the remaining four glycolate
chelators. The position-disordered carbon atoms of glycolates like C5 and C6 in Figure 2 may account for
the resonance splitting of these four glycolates situated in a locally similar chemical environment.
As shown in Figure 2, concerning one monomeric BU NaSi(OCH3)(OCH2CH2O)2, the central
[5]
Si1
atom exclusively adopts an quasi-trigonal-bipyramidal geometry. Oxygens O1 and O3 occupy the axial
positions and form the elongated Si1-O bonds of 1.7656(10) and 1.7422(10) Å, respectively.[32] Oxygens
O2, O4, and O9 take up the equatorial positions at a little shorter Si1-O bond lengths of 1.695(10),
1.6858(10), and 1.6762 (10) Å, respectively. On the basis of the respective atomic coordinates,[32] it is
calculated that the Si1 center resides 0.051 Å below the plane defined by O2, O4, and O9 in reference to
the O1 apex. These bond lengths are in close proximity to the Si-O bond distances found in Refs.,[3,25]
where
[5]
Si complexes with their atomic structures different from this study were reported, but slightly
longer than d(Si-O) of 1.59-1.64 Å for [SiO4] tetrahedra contained in zeolites.[26,27] Partially arising from
inherently asymmetric stereo-configuration of mixed alkoxy ligands, distortions from the idealized
trigonal-bipyramidal geometry towards a square pyramid with O9 at the apex are conceivable, as has been
found for other
[5]
Si compounds. On the other hand, there exist six Na-O contacts in the 2.3032(11)-
2.4106(11) Å scope definitely contributing more or less to such distortions.
5
Packing plots of the B-assisted Si complex viewed along the crystallographic b and [101] axes are
provided in Figure 4 A and B, respectively. By coupling both figures, it can be envisaged that the 3D
crystal structure is constructed of periodically aligned layers parallel to (10ī) plane instead of parallel
chain-like BUs, which agrees well with the above PXRD results. Interestingly, both Si and Na atoms are
in-plane with the layers with their peripherals decorated with the arrays of methanolates and glycolates,
resulting in the interlayers uniquely held together by extensive van der Waals attractive force. The weak
interaction of this type may readily bring about the interplanar slip. In-plane Na+ ions behave both as
charge-counterbalancing cations and cross-linkers, strongly tethering the monomers in each layer together
by strong Na+-O2- ionic bonding interactions. Due to full condensation of the EGs catalyzed by NaOCH3,
the absence of free hydroxyl groups in its structure interprets the lack of identifiable hydrogen bonding
interactions on inter-atoms in the entire crystal lattice.
11
B MAS NMR analysis: SCXRD analysis shows no evidence of B heteroatom incorporation within
the crystal lattice of the B-assisted Si complex. However, one single crystal may not be representative of
the entire sample. Its bulk powders is thus qualitatively analyzed via 11B MAS NMR (data not shown) in
light of the inherent sensitivity for detecting 11B chemical environments due to its high natural abundance
(80.1%). No detectable B species are identified, suggesting the B content is << 1 wt%,[28,29] if any.
ATR-FTIR spectroscopy: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR)
spectra of the B-undoped sample (A), B-assisted one without (B) and with (C) exposure to air for 10 min
are presented in Figure 5. For the B-undoped polycrystalline powders, a broad band at 3000-3600 cm-1
attributable to hydroxyl absorption signal (Figure 5 A) is a combined result of physisorbed moisture and
encapsulated MeOH solvent residues, as corroborated by 13C MAS NMR (Figure 3 C). In contrast, this
weak broad band is significantly enhanced in intensity even subjecting to ambient exposure for 10 min
(Figure 5 B and C), highlighting strong hygroscopicity of the B-assisted Si single crystals. As shown in
Figure 5 A and B, the absorption band at 2750-2980 cm-1 is assigned to the sp3 C-H bond stretching
vibrations, whereas the band at 1340-1500 cm-1 corresponds to the C-H bond bending vibrations. More
specifically, two discernible peaks at 2870/2950 cm-1 (methyl C-H symmetrical/asymmetrical stretching
vibrations) confirm the existence of methyl groups within both Si complexes but with the methyl origin
differing from each other (residual MeOH solvents for the intrinsic sample vs. methanolate ligands for the
B-assisted one). Strong Si-O-C bond stretching vibrations at 933-1110 cm-1 are observed in both samples,
in which the 1030-1070 cm-1 absorption band originating from the C-O bond stretching vibrations
dominates the above spectral range. The absorption band at 600-900 cm-1 is attributed to varying Si-O
bond vibrations including O-Si-O bond vibrational frequency.[30] Any distinguishable peak at ca. 670 cm-1
corresponding to the O-B-O bond bending vibrations[31] does not appear in the B-assisted Si complex
(Figure 5 B and C), revealing the B content below the detection limit of IR spectroscopy.
6
EA: Bulk elemental analysis (EA) is conducted to quantitatively determine the final whereabouts of
the B species. Table 1 tabulates the elemental composition of both Si complexes along with the refined
SCXRD empirical formula for comparison sake. Based upon earlier PXRD and 13C MAS NMR analyses,
Na2Si2(OCH2CH2O)5 formula is assigned to the B-undoped Si complex, which is qualitatively in accord
with the apparent composition of Na1.94Si2C9.80H21.44O10.82. In view of the deliquescence-prone nature of
the B-assisted sample (Figures 5 and 6), the drying step preceding EA is supposed to drive off a small
number of hydrolytically produced MeOH and physisorbed moisture, thus resulting in a slight
underestimation of both carbon and hydrogen contents in relation to SCXRD result. On the other hand,
the finding that the B content is too low to be accurately quantified provides the most direct evidence,
lending further support to the conclusion that the B-assisted Si complex is devoid of identifiable B
incorporation.
TG and DTG analyses: The thermal stabilities of both Si complexes are evaluated by
thermogravimetric (TG) technique both operating in flowing N2 streams, as illustrated in Figure 6. For the
B-undoped polycrystalline powders, between 30 and 370 °C, there exists a minor weight loss (1.2%)
likely corresponding to the evaporation of physisorbed water and residual MeOH solvents. With
increasing temperature, one pronounced weight loss is observed before the final plateau. A dramatic
weight loss associated with the framework collapse is observable over 370-550 °C with a temperature
inflection point at 430 °C. In comparison with the theoretical overall weight loss of 56.4%, the
corresponding value of 53.8% is experimentally obtained while ramping the temperature from RT to
800 °C in an anaerobic environment, ultimately leaving behind the blackened end powders of
decomposition. Namely, after pyrolysis a residual carbon loading of ca. 2.6% for the Si_py composite is
theoretically predicted.
In contrast, the onset degradation temperature of the B-assisted single crystals occurs at 320 °C with a
total weight loss of 2.8% from RT to 320 °C. A larger weight loss is responsible for stronger
hygroscopicity of the B-assisted Si complex relative to the undoped counterpart. In the entire temperature
range, a total weight loss of 52.0% is measured. The theoretical weight loss for the conversion from
Na2Si2C10H22O10 to an oxide of stoichiometry Na2O∙2SiO2 is 54.9%, which means that the Si+B_py
composite theoretically contains a carbon loading of approximately 2.9%. Both materials are hence quite
thermally stable under inert gas atmospheres.
SEM observations: Figures 7 and 8, respectively, show the SEM images of Si_py, Si_py_al, Si+B_py,
and Si+B_py_al, as well as the corresponding magnified images of one single grain in the insets. SEM
images of the intrinsic silica composite (Figure 7 A) indicates that the unwashed sample is composed of
some large irregular grains with a wide particle size distribution. The magnified image (inset) reveals that
the large secondary grain consists of the aggregation of numerous small primary spherical particles (ca.
3.4 μm in mean diameter) and interspersed micro-lamellas, which are tightly held together by the
7
thermally induced sodium silicate binder. After acid leaching, the secondary grains are broken up into
smaller irregular pieces as a result of the dissolution of the binder (Figure 7 B). In contrast, the unwashed
secondary particles of Si+B_py are more uniform and smaller (Figure 8 A) together with the primary
spheres of roughly 1.2 μm in average grain diameter (inset in Figure 8 A). Notably, despite the wide
dispersity of particle sizes, a portion of silica nanoparticles are successfully achieved upon experiencing
simple acid-leaching treatment (Figure 8 B).
Gas sorption analyses on the pyrolyzed products: Figures 9 and 10, respectively, show the N2
sorption isotherms of Si_py, Si_py_al, Si+B_py, and Si+B_py_al, as well as their corresponding PSDs
(insets). Textural parameters for both sets of amorphous silica composite materials are summarized in
Table 2. As seen in both figures, all samples except for Si+B_py exhibit Type IV isotherms with
widespread yet distinct hysteresis loops of Type H3 in the IUPAC classification, characteristic of
mesoporous materials with relatively broad PSDs that are also reflected by the respective PSDs (insets in
Figures 9 and 10). It is found that the sorption isotherms give dramatic increases in SBET from 35.5 for
Si_py to 163.3 m2 g-1 for Si_py_al, and from 30.0 for Si+B_py to 254.7 m2 g-1 for Si+B_py_al. The
corresponding total pore volumes of these four composites are respectively 0.051, 0.211, 0.046 and 0.302
ml g-1. In both cases, the treatment with 2.2 wt% nitric acid solutions leads to remarkably improved
sorption properties in terms of Vt and SBET, which can definitely extract Na+ cations from the pyrolyzed
products with an empirical stoichiometry C(0.4-0.46)∙Na2O∙2SiO2, thereby creating an appreciable volume
fraction of voids occupied by constituent Na2O porogen. Upon acid leaching, the microporosity decreases
from 13.7% for Si_py to 10% for Si_py_al, whereas that increases from 13% for Si+B_py to 20.5% for
Si+B_py_al. The opposite trend can be explained by the inter-particulate void change that occurs over the
course of acid treatment. It is found that acid leaching cannot completely alter the agglomerated
properties of the particles derived from the raw polycrystalline Si complex without any B mediator
(Figure 7 B) in sharp contrast to the resulting better-divided fine powders from the B-assisted single
crystals (Figure 8 B). It is worth noting that acid leaching turns out not to notably modify the mean
mesopore widths (Table 2). The large surface area of these silica composites is principally an outcome of
very small constituent primary spheres (Figures 7 and 8 insets). The coincident carbon deposits even in
significantly low yield are expected to serve as an effective inhibitor minimizing the sintering events to
occur on the growing adjacent silica particles during pyrolysis so as to partially offset the adverse effect
of sodium silicate binder produced simultaneously, thus contributing to the generation of unwashed silica
composites of modest surface area. On the other hand, the resultant silica composites with hierarchical
micro-/meso-porosity could inherit to some degree a legacy of porosity from the corresponding layered Si
complex precursors. Finally, the atomic arrangement featuring the Si centers free of any direct Si-O-Si
linkages (Figure 2) is favorable for the production of high-surface-area silica powders.
8
Conclusion
In this work, we have utilized a modified transesterification mechanism to solvothermally prepare two
well-ordered layered
[5]
Si complexes with similar crystal phase and empirical formula. Additionally, a
novel B-assisted crystallization approach without affecting the elemental kind of the host organosilicate
matrix is presented here allowing for the facile preparation of X-ray quality single crystals, whereas no
any single crystals but polycrystalline powders are directly attained without the aid of B species as an
effective crystal growth modifier. The crystal structure of the B-assisted Si complex with mixed
methanolate and glycolate ligands is for the first time determined by SCXRD technique, highlighting the
major interlayers intercalated with extensive van der Waals gaps even in the presence of substantial Na+
counter-ions due to unique atomic arrangement in its crystal structure. The synthetic strategy proposed
here offers a generalized route for controllable synthesis of diverse layered metal-organic compounds,
which can optionally serve as valuable precursors towards the flexible and facile production of highsurface-area metal oxide composites. Herein, the fabrication of amorphous carbon-silica composites with
an appreciable specific surface area is typically exemplified through rapid pyrolysis under inert
atmosphere, followed by acid leaching. As such, distinct from the classic sol-gel technique, the present
findings afford a new example on how to rapidly synthesize high-surface-area metal oxides. The synthetic
methodology of fast pyrolysis is expected to open up a pathway to effectively prepare such functional
metal oxides with a broad compositional diversity.
Experimental Section
For general methods of characterization and analysis see the SI.
Synthetic procedures: A distinct transesterification synthetic strategy was created to prepare two layered
5-coordinate organosilicate complexes with different empirical formula. No special care was taken to
exclude the exposure to extraneous moisture, and all manipulations were carried out in a well-ventilated
fume hood. All chemicals were purchased from Sigma-Aldrich and used as received without further
purification.
Solvothermal
synthesis
of
the
B-undoped
Si
complex:
A
recipe
of
Si(OCH3)4:3EG:NaOCH3:15CH3OH on a molar basis was originally formulated. In the synthesis, 3.20 g
of sodium methoxide powders (97%) were completely dissolved in 18.14 g of anhydrous methanol
solvent (99.8%) under intense stirring before the addition of 10.81 g of anhydrous ethylene glycol (EG,
99.8%). After thorough homogenization of the aforementioned solution, 8.91 g of tetramethyl
orthosilicate (TMOS, 98%) was rapidly poured into it at room temperature (RT) under agitation.
9
Subsequently, the highly viscous gel thus formed was transferred into an autoclave, and the solvothermal
synthesis was statically carried out at 140 °C under autogenous pressure for 7.5 d. The solid fraction was
recovered via vacuum filtration and repeated rinse with copious amounts of anhydrous methanol. The
collected solid was vacuum dried at 80 °C for 9 h yielding 8.5 g of dry white polycrystalline powders and
then stored in a desiccator for future characterizations.
Solvothermal synthesis of the B-assisted Si complex: To obtain the large single crystals required for
crystal structure elucidation, a small fraction of B species was introduced acting as an effective mediator
between nucleation and crystal growth. A gel precursor with a molar composition of
0.95Si(OCH3)4:0.05B(OCH3)3:3EG:NaOCH3:15CH3OH was prepared by sequentially mixing calculated
amounts of anhydrous methanol, NaOCH3, trimethyl borate (≥ 99%), TMOS and EG under intense
stirring. The resulting thin gel was aged under stirring overnight at RT. The solvothermal synthesis was
statically carried out at 181 °C for 4 d. The solid fraction was collected via vacuum filtration and multiple
rinses with plenty of anhydrous methanol. The recovered solid was vacuum dried at 85 °C for 9 h
affording 3.3 g of transparent diffraction-quality single crystals, e.g., 0.35 × 0.20 × 0.15 mm3 in
dimension.
Pyrolytic synthesis of porous silica composites: Fast pyrolysis operations were conducted on both of
as-synthesized Si complexes in a flowing N2 atmosphere (25 ml min-1) in a TGA furnace using a ramping
rate of 10 °C min-1 up to the targeted 800 °C and a soaking time of 8-10 min. In addition, part of these
pyrolyzed samples were subsequently rinsed overnight at 50 °C under intense stirring with 2.2 wt% nitric
acid aqueous solutions in adequate excess to substantially acid leach the thermally formed Na2O
component. Afterwards, they were collected via vacuum filtration and complete rinse with copious
amounts of deionized (DI) water until the pH of the filtrate was neutral, and finally dried at 120 °C
overnight for future PXRD, SEM, and N2 sorption analyses. The resultant samples were denoted as Si_py,
Si_py_al, Si+B_py, and Si+B_py_al, where py and al represented pyrolysis and acid leaching treatments,
respectively.
Acknowledgements
This research was supported by the U.S. Department of Energy’s Advanced Research Projects AgencyEnergy under control No. 0471-1627. NMR studies were supported by the National Institute of
Biomedical Imaging and Bioengineering of the National Institute of Health under award Nos. EB-001960
and EB-002026. The diffractometer was purchased with the funding from the National Science
Foundation under Grant No. CHE-0946721. V.K.M. is grateful to the Natural Sciences and Engineering
Research Council of Canada for a Postdoctoral Fellowship. We thank Dr. Peter Müller at MIT for
valuable discussion.
10
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12
Entry for the Table of Contents
Entry for the Table of Contents
Transesterification
C
Xiansen Li,* Vladimir K. Michaelis,
Ta-Chung Ong, Stacey J. Smith,
Robert G. Griffin, Evelyn N. Wang*
Designed Single-Step Synthesis,
Structure, and Derivative
Textural Properties of WellOrdered Layered PentaCoordinate Silicon Alcoholate
Complexes
Si
O
Na–O bonding
van der Waals force
Na
1111der Waals gap! Well-ordered
van
layered penta-coordinate silicon
alcoholate is prepared via a new singlestep transesterification mechanism,
whose structure is elucidated with one
single crystal directly acquired by a
novel boron-mediated synthetic route;
these complexes are very effective
precursors for the facile preparation of
high-surface-area amorphous silica
powders.
13
4.0E+06
3.5E+06
Intensity (a.u.)
3.0E+06
C
2.5E+06
2.0E+06
1.5E+06
B
1.0E+06
5.0E+05
A
0.0E+00
0
10
20
30
40
50
2θ (°)
Figure 1. Experimental PXRD
patterns
of as-prepared
B-undoped
Si complex polycrystalline
Figure
1. A: Experimental
PXRD pattern
of B-undoped
powders (A) and numerous B-assisted Si complex single-crystals (B), and simulated pattern of
one single crystal of the B-assisted Si complex (C).
14
Figure 2. Asymmetric unit of the B-assisted crystal structure showing the sodium
tetrakis(glycolato)bis(methanolato)silicate complex Na2Si2(OCH3)2(OCH2CH2O)4.[55] C5 and C6
positional disorders, and hydrogen atoms are omitted for clarity, and thermal ellipsoids are set to
50% probability.
15
Figure 3. MAS NMR of the Si complexes at 16.4 T (697 MHz, 1H): both 29Si (A) and 13C (B) of
B-assisted complex as well as 13C of B-undoped one (C). *transmitter offset.
16
each layer together by strong Na+-O ionic bonding interactions.
B
A
o
ogether by strong Na+-O ionic bonding interactions.
o
B
Figure 4. Pa
o
Figure 4. Packing plots of the B-assisted Si complex viewed along the crystallographic b axis (A)
and [101] direction (B). Hydrogen atoms are omitted for clarity. Blue: Na; Red: O; Gray: C; and
Yellow: Si.
17
1.4
1.2
Absorbance (a.u.)
1
0.8
C
0.6
B
0.4
0.2
A
0
-0.2
600
1600
2600
Wavenumber (cm-1)
3600
Figure 6. ATR-FTIR spectra of the B-undoped Si complex (A), B-doped Si complex without (B)
and with (C) exposure to air for 10 min all measured under ambient conditions
Figure 5. ATR-FTIR spectra of the B-undoped Si complex (A), and the B-assisted one
without (B) and with (C) exposure to ambient air for 10 min.
18
100
2.5
2
80
1.5
70
1
60
Undoped Si complex_TGA
B-assisted Si complex_TGA
Undoped Si complex_DTG
B-assisted Si complex_DTG
50
Derivative weight (%/ C)
Weight loss (%)
90
0.5
40
0
0
100
200
300
400
500
Temperature ( C)
600
700
800
Figure 6. TG and DTG (derivative thermogravimetry) profiles for the B-assisted and Bundoped Si complexes running in flowing N2 atmospheres.
19
A
B
Figure 7. SEM images of Si_py (A) and Si_py_al (B), and an enlarged image of Si_py in the
inset.
20
A
B
Figure 8. SEM images of Si+B_py (A) and Si+B_py_al (B), and an enlarged image of
Si+B_py in the inset.
21
160
Si_py_adsor
Si_py_desor
Si_py_al_adsor
Si_py_al_desor
120
100
80
60
40
Amount adsorbed (ml(STP)/g)
140
20
0
0
0.2
0.4
0.6
Relative pressure (P/P0)
0.8
1
Figure 7. N2 adsorption/desorption isotherms of the pyrolyzed und
Figure 9. N2 adsorption/desorption isotherms of Si_py and Si_py_al, and their
corresponding PSDs in the inset (Si_py: ♦, and Si_py_al: ●).
22
200
150
100
50
Amount adsorbed (ml(STP)/g)
250
Si+B_py_al_adsor
Si+B_py_al_desor
Si+B_py_adsor
Si+B_py_desor
0
0
0.2
0.4
0.6
Relative pressure (P/P0)
0.8
1
Figure 10. N2 adsorption/desorption isotherms of Si+B_py and Si+B_py_al, and their
corresponding PSDs in the inset (Si+B_py: ♦, and Si+B_py_al: ●).
Tables:
Table 1. Elemental analytical and SCXRD-derived results for both Si complexes with and without B mediator (wt%).
Entry
w/o boron
w/ boron
Formula
Na1.94Si2C9.80H21.44O10.82 [a]
Na1.90Si2C8.18H19.11O10.30 [a]
Na2Si2C10H22O10
Source
Expt.
Expt.
SCXRD
B
--< 0.96
0.00
C
28.46±0.028
25.70±0.48
29.67
H
5.24±0.021
5.05±0.13
5.49
Na
10.80±0.023
11.40±0.03
11.37
Si
13.60±1.8
14.70±2.5
13.89
[a] Oxygen content is obtained by difference, and the potential effect of surface carbonate
species on carbon quantity is ignored.
Table 2. Textural parameters for the pyrolyzed samples with and without acid leaching.
dmeso at max.
Sample
Vt (ml g-1)
Vmeso (ml g-1)
Vmicro (ml g-1)
SBET (m2 g-1)
PSD (nm)
Si_py
0.051±0.013
0.044±0.014
0.007±0.004
35.5±1.1
3.82
Si_py_al
0.211±0.033
0.19±0.033
0.021±0.002
163.3±22
3.06
Si+B_py
0.046±0.007
0.040±0.007
0.006±0.002
30.0±1.6
3.84
Si+B_py_al
0.302±0.023
0.24±0.023
0.062±0.002
254.7±8.2
3.83
23
Supporting Information
Designed Single-Step Synthesis, Structure, and Derivative Textural Properties of WellOrdered Layered Penta-Coordinate Silicon Alcoholate Complexes
Xiansen Li,*[a] Vladimir K. Michaelis,[b,c] Ta-Chung Ong,[b,c] Stacey J. Smith,[b] Robert G.
Griffin,[b,c] and Evelyn N. Wang*[a]
[a]
Department of Mechanical Engineering, [b]Department of Chemistry and [c]Francis Bitter
Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
USA
*
To whom correspondence should be addressed.
E-mail: xsli@mit.edu (X. Li) and enwang@mit.edu (E. Wang).
24
1. Methods of characterization
1.1. PXRD: PXRD pattern was taken on the Si complexes and pyrolyzed Si complexes without
dilute acid leaching using a PANalytical X’Pert Pro Multipurpose Diffractometer in reflectance
Bragg-Brentano geometry at 45 kV and 40 mA under Ni-filtered Cu Kα radiation (λ = 1.5418 Å).
The data collection was carried out at a constant temperature of 25 °C with a step increment of
0.084° 2θ, a counting time of 6.4 s/step, and the 2θ angular range from 3 to 50°.
The simulated PXRD pattern of the B-assisted Si complex was based on the .cif file
containing the structural parameters (unit cell size, space group symmetry, and atomic
coordinates, etc.) from the refined SCXRD structure that was imported into a software packageMercury 3.1 which then used these structural parameters to calculate what the PXRD pattern
should look like.
1.2. SCXRD: Low-temperature (-173 °C) XRD data comprising φ- and ω-scans were collected
using a Bruker-AXS X8 Kappa Duo diffractometer coupled to a Smart Apex II CCD detector
with an IμS source of Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct
methods using SHELXS[1] and refined against F2 on all data by full-matrix least squares with
SHELXL-97[2] using established refinement strategies.[3] All non-hydrogen atoms were refined
anisotropically. The positions of all hydrogen atoms were calculated geometrically and refined
using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to
be 1.2 times the Ueq value of the atom to which they were bound (1.5 for methyl groups). The
carbon atoms (C5 and C6) of one of the ethylene glycolate chelators on Si2 are disordered over
two positions (omitted for clarity in Figure 2). Similarity restraints are used for the 1-2 and 1-3
distances for these atoms, and their displacement parameters are constrained to be equal. Rigid
bond and similarity restraints are employed on the displacement parameters for both glycolates
on Si1.
1.3. MAS NMR spectroscopy: Solid-state NMR experiments were performed using either a 9.4
(400 MHz, 1H), 11.7 (500 MHz, 1H), or 16.4 T (697 MHz, 1H) magnet, each equipped with a
home-built NMR spectrometer (courtesy of Dr. D. Ruben, FBML-MIT). Spectra were acquired
using a series of NMR probes including a 3.2 mm home-built double resonance (1H-X, X = 29Si),
a 3.2 mm Chemagnetics triple-resonance (1H/13C/15N), or a triple-resonance (1H/X/Y, X = 11B or
29
Si) MAS probe for 11B, 13C, and 29Si. The white crystalline sample was ground using an agate
mortar and pestle under dry N2 gas and packed into a 3.2 mm (o.d.) high-volume thin-wall ZrO2
rotor (~34 μl fill volume). The magic angle within the probe was set using the 79Br resonance of
solid KBr and shimmed using adamantane prior to signal acquisition. Sample temperatures were
regulated between 22 and 26 °C during acquisition. All spectra were processed using RNMRP
data processing software (courtesy of Dr. D. Ruben, FMBL-MIT).
29
Si MAS NMR spectra (9.4 and 16.4 T, ωL/2π = 78 and 138 MHz, 29Si ɣB1/2π = 50 kHz)
were acquired using either a Bloch or Hahn-echo[4] and a spinning frequency between 10,000 and
11,500(3) Hz, 1,536 and 8,196 co-added transients, and a recycle delay of 60 s, respectively. All
data were acquired with high-power (1H ɣB1/2π = 83 kHz) two-pulse phase modulation (TPPM)
1
H decoupling during acquisition. 29Si spectra were referenced with respect to neat TMS (0
ppm).[5]
13
C MAS NMR spectra (11.7 and 16.4 T, ωL/2π = 126 and 175 MHz) were acquired using
13
C{1H} CP[6] at a ωr/2π = 13,500(3) Hz, 16,384 co-added transients and a recycle delay of 4 s,
respectively. The contact time during the CP experiment was set to 1.5 ms and acquired using
high-power (1H ɣB1/2π = 83 kHz) TPPM[7] 1H decoupling during acquisition. 13C spectra were
referenced relative to solid adamantane at 40.49 ppm (high frequency resonance).[8]
11
B MAS NMR spectra (16.4 T, ωL/2π = 223 MHz) were acquired using a Bloch[9]
experiment with a short quantitative tip angle (15°, 11B ɣB1/2π = 50 kHz), a spinning frequency,
ωr/2π = 16,500(4) Hz, and a series of varying recycle delays (between 3 and 15 s) and co-added
25
transients (between 8,196 and 98,304). 11B spectra were referenced relative to 0.1 M H3BO3
solution at 19.6 ppm with respect to BF3-Et2O (0 ppm).[10]
1.4. ATR-FTIR spectroscopy: IR absorbance spectra were collected using a Nexus 870 FT-IR
E.S.P. spectrometer (Thermo Scientific) equipped with an ATR accessory with a single
reflection diamond crystal. The FT-IR chamber was flushed constantly with N2 (ca. 0.71 m3 h-1).
Prior to data collection, the chamber was purged with flowing N2 stream for at least 15 min.
Scans at a spectral resolution of ±4 cm-1 were taken at RT on a self-supporting sample disc from
600 to 4000 cm-1. Sixteen scans were averaged, and the resulting spectra were background
subtracted.
1.5. Elemental analysis: For bulk elemental analysis, the samples were sent to Galbraith
Laboratories, Knoxville, TN. They were dried at 70 °C for 4 h in vacuo before quantifying
carbon and hydrogen elements. Elemental analyses were performed for silicon, sodium, and
boron as well. The content of oxygen was obtained by difference.
1.6. TG analysis: The sample of Si complex with an initial mass of several tens of mg was
heated in a Pt pyrolytic pan at a constant ramping rate of 10 °C min-1 from ambient temperature
up to 800 °C on a Discovery TG analyzer from TA Instruments in a flowing N2 atmosphere
preset to 25 ml min-1. The isothermal duration at 800 °C was set to 3 min. The experimental
corrected sample standard deviations in TGA analyses were 0.034% and 0.18% for the Bassisted and B-undoped Si complexes, respectively.
1.7. SEM: The morphology of the pyrolyzed silica particles before and after dilute acid rinses
was characterized by an Analytical Scanning Electron Microscope (JEOL-6010LA) at an
accelerating voltage of 10 or 15 kV. A gold film was sputter-coated onto these samples before
imaging.
1.8. Gas sorption analysis: Gas sorption studies were conducted to investigate the textural
properties of the pyrolyzed products with and without a dilute acid leaching treatment. The N2
sorption measurements were performed at -196 °C using an Autosorb iQ2 automated gas sorption
analyzer (Quantachrome). Before adsorption runs, each sample was degassed under vacuum (ca.
0.2 Pa) at 370 °C for 12 h, and a proper glass rod filler was then inserted in the corresponding
sample cell to minimize the cell dead void. The BET (Brunauer, Emmett, and Teller) surface
area, SBET, was obtained by applying the BET equation to a relative pressure (RP) range of 0.050.30 on the adsorption branch. The total pore volume, Vt, was evaluated from the adsorbed N2
amount at a maximal RP of 0.95. The micropore volume, Vmicro, was determined by applying the
D-R (Dubinin-Radushkevich) equation to a RP range of 0.0003-0.009 on the adsorption isotherm.
The pore size distribution (PSD) was calculated by the BJH (Barrett, Joyner, and Halenda)
method on the desorption branch.
2. Experimental Results
Table S1. Crystal data and structure refinement for the B-assisted Si complex.
Parameter
Data
Identification CCDC No.
965918
Empirical formula
C10H22Na2O10Si2
Formula weight
404.44
Temperature
100(2) K
Wavelength
0.71073 Å
Crystal system
Monoclinic
Space group
P2(1)/n
26
Unit cell dimension
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 31.50°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
a = 8.7279(5) Å
α = 90°
b = 15.0981(9) Å
β = 96.8920(10)°
c = 13.7362(9) Å
γ = 90°
1797.00(19) Å3
4
1.495 Mg m-3
0.291 mm-1
848
0.35 × 0.20 × 0.15 mm3
2.01 to 31.50°
-12 ≤ h ≤ 12, -22 ≤ k ≤ 20, -20 ≤ l ≤ 20
54664
5975 [R(int) = 0.0306]
100.0 %
Semi-empirical from equivalents
0.9577 and 0.9051
Full-matrix least-squares on F2
5975/112/226
1.067
R1 = 0.0370, wR2 = 0.1050
R1 = 0.0428, wR2 = 0.1093
1.364 and -0.587 e Å-3
3.0E+05
2.5E+05
Intensity (a.u.)
2.0E+05
C
1.5E+05
B
1.0E+05
5.0E+04
A
0.0E+00
12
22
32
2θ( )
42
Figure S1. Experimental PXRD patterns of as-prepared B-undoped Si complex polycrystalline
powders (A) and numerous B-assisted Si complex single-crystals (B), and simulated PXRD
pattern from single-crystal X-ray data of the B-assisted Si complex (C). Three XRD patterns are
all magnified in the high-angle region.
3. References
[1] G. M. Sheldrick, Acta Cryst. 1990, A46, 467-473.
[2] G. M. Sheldrick, Acta Cryst. 2008, A64, 112-122.
[3] P. Müller, Crystallography Reviews 2009, 15, 57-83.
[4] E. L. Hahn, Phys. Rev. 1950, 80, 580-594.
[5] R. K. Harris, E. D. Becker, J. Magn. Reson. 2002, 156, 323-326.
[6] A. Pines, J. S. Waugh, M. G. Gibby, J. Chem. Phys. 1972, 56, 1776-1777.
27
[7] A. E. Bennett, C. M. Rienstra, M. Auger, K. V. Lakshmi, R. G. Griffin, J. Chem. Phys. 1995,
103, 6951-6958.
[8] C. R. Morcombe, K. W. Zilm, J. Magn. Reson. 2003, 162, 479-486.
[9] F. Bloch, Phys. Rev. 1946, 70, 460-475.
[10] R. K. Harris, E. D. Becker, J. Magn. Reson. 2002, 156, 323-327.
28